This invention relates to mattresses, to mattresses for beds and to mattresses that efficiently provide low pressure and alignment to achieve restful and less fragmented sleep.
Normally, everyone spends a large percentage of everyday sleeping. Restful sleep is important to a person's good health, enjoyment of life and the ability to function normally. Sleep affects brain activity, heart rate, blood pressure, sympathetic nerve activity, muscle tone, blood flow, sexual arousal, body temperature and other body conditions. Poor sleep has a strong correlation to obesity, diabetes, stroke, depression, hypertension and other adverse conditions.
Restful sleep is dependent upon a person's comfort level while recumbent, usually in side-lying and back-lying positions. The concentration of pressure on certain parts of the body and poor body alignment are significant causes of restless sleep.
During sleep, a healthy person typically passes through four levels of sleep which include physically restorative stages I-III and which additionally includes a REM (Rapid Eye Movement) sleep stage, the mentally restorative stage. Stages I and II are the lightest sleep and stage III is the deepest, the Stages I, II and III are non-REM stages (NREM). The REM stage is that level in which sleepers dream and receive mental health benefits. All levels of sleep are important, but stage III is the deepest and most physically restful sleep, when, for example, human growth hormone is secreted. Normal sleep cyclically passes through the stages from I to III and back from III to I and into and out of REM. This sleep cycle is repeated a number of times over a normal sleep period, but can be disrupted due, for example, to body discomfort.
Restfulness and the quality of sleep (mentally and physically restorative sleep) are dependent upon the comfort of sleepers. When sleepers become uncomfortable, they move to relieve the discomfort and the resulting moves are a normal part of sleep. When sleepers move, they frequently change to lighter stages of sleep or awaken. The more discomfort sleepers feel, the more they will move and the more time they will spend in lighter and less restful sleep. Good sleeping is normally associated with a minimum number of interruptions of sleep stages due to a low number of body shifts during the sleep period. The higher the number of interruptions, the more fragmented the sleep and the less restful the sleep.
Comfortable mattresses are important in establishing restful sleep. Bed-induced shifts due to discomfort caused by the bed are a significant cause of poor sleep quality. On conventional mattresses (including feather beds, inner spring mattresses, foam mattresses, orthopedic mattresses, waterbeds, airbeds and the like), most people experience as many as forty major postural body shifts in the course of a night's sleep. Poor sleepers experience as much as sixty percent more major shifts than good sleepers. While some shifts during a sleep period are beneficial, the quality of sleep can be greatly improved for many by reducing the number of bed-induced shifts.
There are two major causes of bed-induced shifting that cause poor sleep. The first major cause of shifting is excessive pressure on parts of the body and the second major cause of shifting is the body's spinal misalignment.
Considering the first major cause of shifting, the buildup of pressures results from prolonged lying in the same position. On conventional mattresses, the pressure tends to be greatest on the body's protrusions (such as shoulders and hips) where body tissues are put in high compression against the mattress. High compression tends to restrict capillary blood flow which is recognized by the body, after a period of time, as discomfort. The amount of pressure which causes a discontinuance of capillary blood flow is called the ischemic pressure. The ischemic pressure threshold is normally considered to be approximately thirty mmHg. The discontinuance of capillary blood flow is observable as a red spot on the skin (reactive hyperemia). After pressure is applied, a red spot on the skin is a precursor to tissue damage. When parts of the body (usually shoulders and hips in conventional mattresses) are subjected to pressures above the ischemic threshold, discomfort results and, hence, a person shifts to remove the discomfort and threat to tissue damage.
Considering the second major cause of shifting, body misalignment results from spinal misalignment due to lateral bending of the vertebral column of the body, particularly for a person in a side-sleeping position. Such lateral bending is typically caused by mattresses that allow sagging of the torso region of the body. Conventional mattresses allow such sagging regardless of the hardness or the softness of the mattress but the spinal sagging effect tends to be more pronounced on firm mattresses. A sagging mattress allows the upper torso (thoracic region) to drop relative to the hips and results in stress to muscles and ligaments. The stress from a sagging mattress frequently manifests as discomfort or even pain in the lumbar region of the back. Such discomfort causes the sleeper to shift in order to relieve the discomfort and avoid tissue damage.
Similarly, when lying in the supine position, the hips form a higher support point than the lumbar region of the spine. A flattening of the lumber spine due to gravity then occurs and this, again, brings stress to the soft tissues and causes a turning away from this position to avoid discomfort and tissue damage.
In U.S. Pat. No. 6,807,698, a bed having low body pressure and alignment includes a mattress for supporting a recumbent body. The mattress includes a resilient top member having a top region possessing uniform placement parameters and also includes resilient supporting means supporting the top member with variable displacement. The combination of members with uniform displacement parameters over members with variable displacement parameters enables the mattresses to support the body in alignment and with uniform low pressure.
In U.S. Pat. No. 7,036,172, a bed having low body pressure and alignment includes a mattress supporting a recumbent body with low body pressure and in alignment. The mattress extends in a lateral direction from side to side and extends in a longitudinal direction from a mattress head to a mattress foot where the mattress includes a head part, a shoulder part, a thoracic part, a hip part and a leg part. The recumbent body has a displacement profile that causes the mattress to undergo differing displacements when supporting the recumbent body. The mattress composite has displacement parameters varying to match the displacement profile of the recumbent body while supporting the recumbent body with low body pressure. The composite has a plurality of regions where the displacement in one or more of the regions varies to match the displacement profile of the recumbent body to maintain the recumbent body in alignment.
An ideal mattress has a resiliency over the length of a body on the mattress to support the body in spinal alignment and also has a low surface body pressure over all or most parts of the body in contact with the mattress. Since a recumbent body has both varying density and varying contour in the longitudinal direction, the ideal mattress must conform to these variations. With such variations, in order to achieve spinal alignment, the supporting forces in the mattress, under load from the recumbent body, must vary along the body to match the varying body density and shape. Also, when the body is in spinal alignment, for an ideal mattress, the supporting pressures in the mattress against the skin must be low. The preferred pressure against the skin of a person in bed for an ideal mattress is generally below the ischemic threshold. The preferred side-lying spinal alignment for a person in bed is generally defined as that alignment in which the spine is generally straight and on the same center line as the legs and head, a condition that helps provide “spinal neutrality”. “Spinal neutrality” is a condition in which the forces on the spine and ligaments have minimum stress, for example, the shear forces on the L1 and L5 vertebrae are a minimum.
While the general principles of an ideal mattress have been recognized, actual embodiments of mattresses that have properties that approach the properties of an ideal mattress at reasonable costs have not been fully satisfactory.
Developments in the parameters of and manufacturing capabilities for foam and other materials have provided new components for mattresses that can be used to better approach the technical parameters required for an ideal mattress at economical costs and which can be manufactured with expected standard properties and with the attributes for mattresses that are desired by the public.
There are a number of properties useful in characterizing mattress materials including “Hardness”, “Density”, “Indentation Load Deflection (ILD)” and “Tensile Strength”. Hardness is the resistance against pressure. Density is the mass per unit volume. Hardness and density are interrelated. When density increases, hardness tends to increase. Generally for lower density materials, a growing loss in hardness arises after repeated loading. Tensile Strength is the measure of the resistance against stretching and changes in tensile strength are measured as Tensile % and changes in length after applying a tensile force are measured as Elongation %. Indentation Load Deflection (ILD) is a hardness measurement defined in the ISO 2439 standard. ILD in the standard is defined as the force that is required to compress material a percentage of its original thickness, that is, compressed 25%, 40% and 60% from its original thickness (using in the standard a circular plate of 322 cm.sup.2). These ILD's are designated ILD.sub.25%, ILD.sub.40% and ILD.sub.60%.
In consideration of the above background, there is a need for improved mattresses that better approach the properties of ideal mattresses and that can be economically manufactured while satisfying the public expectations and demands for mattresses.